There is no federal law prohibiting states from allocating electoral votes with no regard for the popular vote. But these electoral votes are allocated by state governments, which tend to be democratic in nature. States could make laws allocating electoral votes in an authoritarian manner, but politicians who support such legislation would likely lose the support of their largely pro-democracy constituents, and lose their position of power.
In order to effectively subvert democracy, you need to keep the wool over the population’s eyes until it’s too late for them to do anything. It’s kind of hard to do that with something like this.